Review

• Destinations for proteins synthesized on free ribosomes in the cytosol

  • Nucleus
  • Mitochondria
  • Chloroplasts
  • Peroxisomes
• Destinations for proteins synthesized on ER ribosomes


  • Golgi apparatus
  • Lysosomes
  • Plasma membrane
  • Extracellular space
• Sorting signal(s) or signal sequence

  Segment(s) in the amino acid sequence that direct proteins to their intended destinations
• Sorting receptors


  Proteins that recognize sorting signals and guide newly-synthesized proteins into the appropriate compartment
• Signal peptidases


  Enzymes that cleave off and remove signal sequences from newly-synthesized proteins as they reach their final destinations
• Newly-synthesized proteins are fully folded before transport to the nucleus
• Nuclear localization signals


  Specific amino acid sequences that target proteins to the nucleus
• Nuclear transport receptors


  Receptors that recognize nuclear localization signals
• Nucleoporins


  Proteins that control the bidirectional trafficking of proteins traversing the nuclear pores, facilitating regulated transport between the nucleus and the cytoplasm
• Exportin

  Receptors that mediate nuclear export of proteins, recognizing sorting signals and transporting proteins through the nuclear pore into the cytosol
• TIM complex translocon

  Translocates preproteins into the inner membrane and across the inner membrane into the mitochondrial matrix
• Newly-synthesized proteins are unfolded during translocation to the mitochondria, and then folded in the mitochondria with the help of molecular chaperones
• Organelles that form the secretory pathway

  • Endoplasmic reticulum
  • Golgi apparatus
  • Lysosomes
• Disulfide bonds


  Formed between the side chains of cysteine residues of a newly synthesized protein within the endoplasmic reticulum
• Protein modifications that occur in the Golgi apparatus

  • Glycosylation
  • Sulfation
  • Phosphorylation
• Vesicular transport

  The mechanism that facilitates the transport of proteins from the ER to their final destination